Many technological applications today use semiconductor devices. These devices are the product of extensive research and development efforts in many areas including such areas as semiconductor epitaxial growth and device design. Many devices use only a single semiconductor material, for example, silicon, which may be used to fabricate a variety of devices including transistors, photodetectors, etc.
Devices have also been proposed which use several semiconductor materials having different bandgaps. One such device is the heterojunction bipolar transistor first disclosed by Shockley. More recently, devices have been developed which have pluralities of interleaved layers with different energy bandgaps to obtain desirable device characteristics. Such devices include, for example, multi-quantum well and superlattice structures. These structures are useful in many type of diverse devices including light sources and detectors as well as devices exhibiting negative differential conductivity. See, for example, Applied Physics Letters, 40, pp. 38-40, January 1982, which describes a superlattice photodiode. In these structures, the energy bandgap typically varies from layer to layer but is relatively constant within any given layer.
Still more recently, devices have also been proposed and developed having a graded energy bandgap within a layer, i.e., the bandgap within a layer varies as the, e.g., composition or doping concentration varies. See, for example, U.S. Pat. No. 4,383,269 issued on May 10, 1983 to Federico Capasso which describes an avalanche photodetector having a graded bandgap that enhances the ionization rate of one type of carrier with respect to the other type of carrier. Another such structure, which may be termed a sawtooth structure, may be used as a rectifier. Yet another structure, termed a staircase, when suitably biased, is useful as a semiconductor photodetector which is the solid state analog of a photomultiplier. See, for example, IEEE Transactions on Electron Devices, ED-30, pp. 381-390, April 1983. A sawtooth or staircase structure has an energy bandgap that increases monotonically, within a layer, from a lower value to a higher value followed by a stepback to a lower value at the interface with the next layer. When the change, i.e., stepback, in the energy bandgap is sufficiently large in the staircase, at least one type of carrier acquires sufficient energy to impact ionize. The term sawtooth or staircase thus refers to the energy bandgap structure. The graded bandgap layer, or layers, is sandwiched between two epitaxial layers which have the same and opposite conductivity types for the rectifier and photodetector, respectively. When used as a rectifier, the two epitaxial layers have a relatively narrow bandgap as compared to the graded layer. For the rectifier application, a bias voltage must be applied. See, e.g., U.S. Pat. No. 4,353,081 issued on Oct. 5, 1982 to Allyn, Gossard and Wiegmann.
A device using relatively thin highly doped layers to obtain a triangular barrier is described in Electronics Letters, 16, pp. 836-838, Oct. 23, 1980. The resulting majority carrier device was stated to be useful as a rectifying device and as a mixer diode.